Electronic structure of a realistic STM tip: the role of different apex atoms

In view of interpreting images obtained by scanning tunnelling microscopes with perturbation methods the electronic properties of the STM tip are of u tmost importance. We have calculated with a first-principles method the spi n-resolved electronic structure of a tungsten (100) surface with different apex atoms. The results presented show that the electronic states in the va cuum region are in each case considered a mixture of states with s-, p- and d-type symmetries. The relevance of this result for STM imaging and a calc ulation of the tunnelling current by numerical methods is discussed briefly .